1. Field of the Invention
The present invention relates a transfer device that transfers a visualized image formed on an image carrier onto a recording medium, and an image forming apparatus including the transfer device.
2. Description of the Related Art
An electrophotographic image forming apparatus forms an image by visualizing a charged latent image obtained by imaging optical image information onto an image carrier that has been evenly charged in advance, using toner supplied from a developing unit, and by transferring and fixing the image which is thus visualized onto a recording sheet (recording medium). In such an image forming apparatus, because a recording sheet has some texture, toner is less easily transferred onto recessed parts than projected parts. In particular, when toner is to be transferred onto a recording sheet with a highly textured surface, the toner might not be transferred well onto recessed parts, and might result in white splotches in the image.
As a countermeasure for this issue, Japanese Patent Application Laid-open No. 2006-267486, Japanese Patent Application Laid-open No. 2008-058585, and Japanese Patent Application Laid-open No. H9-146381, for example, describe technologies for improving a transfer ratio by superimposing an alternating current (AC) voltage on a direct current (DC) voltage.
The technology disclosed in Japanese Patent Application Laid-open No. 2006-267486 performs control using an AC voltage superimposed on a DC voltage as a transfer bias, and charging the surface of a recording sheet to the opposite polarity of that of the toner in a manner suitable for the texture before transferring the image so that toner is to be transferred to recessed parts.
The technology disclosed in Japanese Patent Application Laid-open No. 2008-058585 uses an AC voltage superimposed on a DC voltage as a transfer bias. The AC voltage is superimposed in a manner making the voltage between peaks of the AC voltage equal to or less than twice the DC voltage.
The technology disclosed in Japanese Patent Application Laid-open No. H9-146381 uses fluorine resin on the surface of an intermediate transfer element, and uses an AC voltage superimposed on a DC voltage as a transfer bias. The AC voltage is superimposed in a manner making the voltage between peaks of the AC voltage equal to or more than 2.05 times the DC voltage.
Although all of these technologies attempt to improve transferability by controlling voltages applied from the DC power source and the AC power source to the target values, detailed descriptions in these disclosures merely disclose the relations between the transfer voltage and the transferability.
In a transfer device for improving the toner transferability by superimposing a DC voltage on an AC voltage and applying the resultant voltage to recessed parts of textured paper, depending on the output AC voltage and DC voltage, the density in smooth parts, the transferability in the recessed parts, and abnormalities of images resulting from discharge may vary. Therefore, the AC voltage setting and the DC voltage setting need to be kept within a certain range. However, it is also necessary to change the ranges of the AC voltage setting and the DC voltage setting depending on a change in the resistance in the transfer member caused by environmental changes, e.g., a change in temperature or humidity, or depending on the type of a paper sheet that is a recording medium. In the method in which a DC voltage is superimposed on an AC voltage and the resultant voltage is applied, the acceptable ranges of the voltage settings are more limited for the aforementioned reason than those in a conventional transfer device applying only a DC voltage. Furthermore, the relation between the DC voltage, the AC voltage, and the resultant image is complex. Therefore, it is difficult to cope with resistance changes and different types of paper sheets.
There is a need to address the issue described above in conventional transfer units, and an object of the present invention is to provide a transfer unit and an image forming apparatus that improve the transfer ratio to the recessed parts on a textured surface of a paper sheet, that can transfer toner evenly even to a paper sheet having a highly textured surface, and that can output high quality images in a stable manner even in environmental changes and for different types of paper sheets.